In semiconductor device manufacturing, photolithography is typically used to transfer a pattern for forming semiconductor features onto a semiconductor process wafer for the formation of integrated circuits. During a photolithographic process, radiant energy such as ultraviolet light is passed through a photomask, also referred to as a reticle, to expose a radiant energy sensitive material such as photoresist formed on the wafer process surface. The mask includes predetermined circuitry patterns having attenuating regions and non-attenuating regions where the radiant energy is modulated in both intensity and phase. In a typical process, exposed portions of the photoresist are developed to form a pattern for subsequent processes such as etching of features into underlying material layers.
As semiconductor device feature sizes have decreased to sizes smaller than the wavelength of light used in photolithographic processes, optical fringing of light passing through a photomask increasingly becomes a problem in forming features with small critical dimensions (CDs), for example less than about 0.25 microns. Light passing through different portions of a photomask causes constructive and destructive interference effects, also referred to as optical fringing or diffraction, which causes undesired light exposure on the photoresist in undesired places. As a result, a loss of pattern resolution in transferring the reticle pattern to the photoresist occurs.
To increase the resolution of a transferred photolithographic pattern, phase shift masks (PSMs) have been developed where the phase of the wavefronts of light passing through the reticle pattern are intentionally phase shifted in selected portions to selectively produce destructive interference thereby reducing undesired light exposures of the photoresist of the photoresist due to diffraction of light passing through the patterned reticle (mask). As a result, the contrast, and therefore, the transferable resolution of the patterned reticle is improved.
There have been several different types of masks developed to improve resolution for different types of mask patterns. For example, in an attenuated or halftone phase shift mask, the phase shifting function is typically accomplished by adding an extra layer of partially transmissive material to the mask with predetermined optical properties. Some PSMs are designed to produce improved resolution while having little improvement in depth of focus, while other PSMs are designed to have relatively modest increases in resolution while producing a greater improvement in depth of focus. For example, attenuated PSMs, also referred to as halftone PSMs, are of the latter type.
In a conventional mask forming process, an opaque layer is typically formed overlying a phase shifting layer. A first photoresist layer is formed and patterned over the opaque layer, followed by etching the pattern into the opaque layer. Following removal of the first photoresist layer, the patterned opaque layer is then used as a hardmask to etch the phase shift layer. A second photoresist layer is then formed over the patterned phase shift and opaque layers, followed by a second patterning and etching process to remove portions of the patterned opaque layer to form a phase shift mask (PSM) including a circuitry pattern.
Problems with prior art PSM formation processes include the necessity of mask alignment and exposure of the PSM in more than one photolithographic patterning process, thereby increasing the probability of optical misalignment and the formation of defects in subsequent etching processes. Moreover, the necessity of more than one photolithographic patterning process to produce the PSM, including associated exposure, development, and removal processes, contributes to a lengthy and therefore costly process flow.
Thus, there is a need in the semiconductor manufacturing art for an improved PSM and method of forming the same to reduce manufacturing defects while improving a process flow.
It is therefore among the objects of the present invention to provide an improved PSM and method of forming the same to reduce manufacturing defects while improving a process flow, in addition to overcoming other shortcomings and deficiencies of the prior art.